1. Field of the Invention
The present invention relates to optical networks, signal routing network elements for such networks and methods for implementing optical networks. Especially the invention concerns an all-optical network, passive wavelength routers for such networks, and methods for implementing all-optical networks. Uses of the invention are also disclosed.
2. Discussion of the Background
An optical network consists basically of nodes with optical transmitters and receivers, optical fibre as the transmission line connecting these nodes, and signal routing/switching network elements (NE) at the nodes. An all-optical network is a network where the signal remains in the optical domain all the way from the transmitting to the receiving end, i.e. where no conversion from the optical to the electrical domain (or vice versa) takes place at the intermediate nodes. In today""s backbone network the transmission between nodes is done optically, yet the network itself is actually not all-optical in that all the processing and routing functionality at the node is done in the electrical domain. The signal is converted to the electrical domain at each node and the header information that is incorporated in the digital signal serves to direct it forward to the next node in order to reach its final destination. In this respect all the routing, i.e. the path-finding, through the network is done at levels other than the optical. The introduction of optical add-drop multiplexers (OADM) makes it possible to connect some nodes directly, by-passing that is some of the intermediate nodes and creating a direct optical path for a certain signal to reach the node it is destined for. However, this is far from the vision of the all-optical network where all the routing functionality will be done in the optical domain. The implementation of all-optical networks will have significant consequences that have been extensively analysed within the field of optical communications. The introduction of optical network functionality at the node is the major area of interest in the field of optical communications and the realisation of good performance all-optical or nearly all-optical networks is the main motivation behind nearly all of the work carried out in the field.
Wavelength Division Multiplexing (WDM) is used to multiplex many optical channels in one fibre. At the receiving end the signal is divided back to its original constituents and each wavelength is received on a separate receiver that is accordingly tuned to the right frequency/wavelength. The number of channels in a WDM transmission system has increased dramatically in the past three years. Fixed wavelength transmitters are used and WDM systems with two hundred WDM channels in one fibre, have been announced by systems producers. WDM is today used for capacity increase yet the real driving force behind its tremendous development is the prospect of optical networking. The wavelength of the channel is used to identify the signal in the optical domain so that it can be directed as necessary without the need for demultiplexing and reading of the content of the signal itself.
One of the main building blocks of the (all-) optical WDM network is the optical cross-connect (OXC). This is a controllable NE that is used at the node to direct optical signals to the right output. Any optical signal that arrives at a certain fibre input to the node will be switched over to the fibre output that will ensure that this signal follows the right path towards its end destination. A typical OXC ought then to be able to switch each one of those (100) wavelengths from each one of the input fibres (minimum of two) to the right fibre output (between a minimum of two output fibres). OXCs are just becoming commercially available, primarily electro-optic versions of these with far from ideal performance and quite high cost. The sheer size, cost and complexity are such that it has not been proved possible to realize an OXC of admissible specifications/performance for a real implementation. The complexity and cost of an OXC increases dramatically as a function of size, the size being defined as a function of the total number of wavelengths as well as the number of fibre inputs and outputs. In addition, unrealistically stringent performance requirements are placed upon the components the OXC (e.g. optical switches). It is quite likely that better OXC may be realized in the future, yet it has become clear that technical limitations will render it practically impossible to realize a large high capacity all-optical network that comprises cascades of large OXCs. This constitutes a considerable limitation for the realisation of optical networks which has been a major delaying factor to their introduction.
Recently, a hard-wired OXC that functions as a passive wavelength router was described. See Chen et al., xe2x80x9cFiber Bragg Grating-Based Large nonblocking Multiwavelength Cross-Connects,xe2x80x9d Journal of Lightwave Technology, Vol. 16, NO. 10 (October 1998), the entire content of which is hereby incorporated by reference. In this device, each wavelength from each input is directed to a predetermined output, and at the same time, signals carried by the same optical wavelength originating from different inputs are directed to distinct outputs , a functionality that is central in a cross-connect as indeed its name implies.
An object of the present invention is to provide a method for the realisation of a network where no switching elements, such as the OXC or others, are included at the node.
Some known networks can be considered switchless network. For example, a simple ring network with non configurable OADMs is a switchless network. In the case of a star network, a star wavelength demultiplexer has been used to provide dedicated wavelength connections from node to node. These two network architectures can be used in the access or the regional area network. However, in the backbone network area and for larger scale networks, mesh architectures are practically required. Star networks suffer unacceptable protection problems to be implemented in the backbone area where full protection is required and make a very inefficient use of the available fibre infrastructure. OADM rings offer low functionality in relation to a mesh, resulting in long transmission lengths and a rather wasteful use of the optical bandwidth. In order to realize a mesh optical network, an OXC is generally required exactly because many choices of direction are present for each signal at each node. One of the main contributions of the present innovation is to devise a method for the realisation of a switchless mesh network with the possibility to attain the performance that is required from today""s networks. This features simultaneous connections between all node-pairs, high bandwidth between the nodes, 1+1 (1:N) protection, dynamic bandwidth allocation, and efficient use of the bandwidth available in the fibre. Obviously, lower performance is also attainable.
In accordance with a first aspect of the invention there is provided an optical network comprising a plurality of nodes, optical fibre transmission lines connecting the nodes, at least one transmitter and/or at least one receiver in each node, and at least one wavelength router. The router is operative to route at least one wavelength band that contains more than one consecutive optical channel. The optical network can be a mesh, a ring, a star or a bus network, or combinations of these.
In accordance with a second aspect of the invention there is provided an all-optical network comprising a plurality of nodes, optical fibre transmission lines connecting the nodes, at least one transmitter and/or at least one receiver in each node, and at least one passive wavelength router, wherein the network is a transparent/passive medium with a set of fixed wavelength dependent rules, with a plurality of paths connecting the nodes, the network being a mesh network, a path of an optical signal propagating through the network being determined at the transmitter by choosing the transmitted wavelength of the optical signal, the paths of the network being predetermined according to the set of fixed wavelength dependent rules.
In a preferred embodiment the transmitters in at least some of the nodes are tunable lasers. The receivers in at least some of the nodes can be fixed wavelength receivers or tunable wavelength receivers.
The set of fixed wavelength rules comprises preferably restrictions of the directions of a transmitted optical signal, first in purely geometrical/spatial terms and second as dependent upon the wavelength of the signal.
In another preferred embodiment the router is a wavelength band router, and wavelength bands are then routed around the network. To provide reconfigurability of the wavelength routers arranged in at least some of the nodes, at least one switch may be arranged in these nodes. It is also possible to increase the wavelength reuse and the flexibility of the network if at least one wavelength converter is arranged in at least some of the nodes.
In another preferred embodiment of the all-optical network according to the invention, two optical fibres can be arranged between each pair of neighbouring nodes, the routers providing two physical layers in the network, each physical layer including preferably all of the nodes, to provide bidirectional connection between all of the nodes in the network, the transmission in each fibre being unidirectional.
One may also provide multiple optical fibres between each pair of neighbouring nodes, the routers providing multiple physical layers in the network, each physical layer including preferably all of the nodes, to provide bidirectional: connection between all of the nodes in the network, the transmission in each fibre being unidirectional.
The transmission in the fibres in the network may be bidirectional.
In accordance with a third aspect of the invention there is provided a passive wavelength router for an optical network, wherein at least one combiner for combining a number of input optical signals, and at least one circulator for providing a number of input optical signals to a number of outputs, reflective filters being implemented at a number of the outputs. Alternatively, a passive wavelength router may include a circulator for receiving a single input and a power combiner that combines several outputs. In this case, each input to the node is routed to several outpouts using passive wavelength filters. For routing more than one input into several outputs, several of such single-input-circulator/power-combiner blocks can be combined.
For illustration purposes, the embodiments of the present invention are discribed herein using circulators. However, the function performed by the examplary circulator can be performed by various combinations of couplers, as would recognize a person of ordinary skill in the art. Therefore, the present invention is not intended to be limited to using circulators in the routers described herein, but is intended to include various combinations of couplers that perform an equivalent function.
In accordance with a fourth aspect of the invention there is provided a passive wavelength router for an optical network providing bidirectional connection between nodes in the network, the transmission in each fibre being unidirectional, wherein the router comprises a first and a second single layer router, each single layer router comprising at least one combiner for combining a number of input optical signals, at least one circulator for providing the input optical signals to a number of outputs, reflective filters being implemented at a number of outputs, wherein various inputs and outputs of the first and second single layer routers are connected.
In accordance with a fifth aspect of the invention there is provided a passive wavelength router for an optical network providing bidirectional connection between nodes in the network, the transmission in each fibre being bidirectional, wherein the router comprises a number of single layer routers and a number of circulators, each single layer router comprising at least one combiner for combining a number of input optical signals, at least one circulator for providing a number of input optical signals to a number of outputs, wavelength reflective filters being implemented at a number of the outputs, wherein various inputs and outputs of different single layer routers are connected.
In accordance with a sixth aspect of the invention there is provided a passive wavelength router for an optical network providing preferably bidirectional connection between nodes in the network, wherein the router comprises a number of single layer routers and a number of circulators, each single layer router comprising at least one combiner for combining a number of input optical signals, at least one circulator for providing a number of input optical signals to a number of outputs, wavelength reflective filters being implemented at a number of outputs, wherein various inputs and outputs of different single layer routers are connected by simple fibre connections or via additional circulators.
The routers are preferred to be operative to route wavelength bands and the wavelength reflective filters are preferably broadband wavelength reflective filters or broadband Bragg filters. The combiner is in the general case a simple power combiner but may also be wavelength dependent.
In accordance with a seventh aspect of the invention there is provided a method for implementing an all-optical network, the network comprising a plurality of nodes; optical fibre transmission lines connecting the nodes; at least one transmitter and/or at least one receiver in each node, and a passive wavelength router; the network being a passive medium with a set of fixed wavelength dependent rules, the network being a mesh network with a plurality of paths connecting the nodes. The method including the step of determining the path of an optical signal propagating through the network by choosing at the transmitter the transmitted wavelength of the signal, the paths of the network being predetermined according to the set of fixed wavelength dependent rules.
The set of fixed wavelength dependent rules comprises preferably restricting the directions of a transmitted optical signal, first in purely geometrical/spatial terms and second depending upon the wavelength of the signal.
Preferably wavelength bands are routed in the network.
Time-division multiplexing may be implemented by rapid switching of the transmitter wavelength, allocating a time slot to each path handled by said transmitter. The transmission of internet protocol (IP) packets and transmission of asynchronous transfer mode (ATM) cells through the network may also be provided with the method of the invention.
By introducing at least one switch in at least some of the routers a certain re-configurability of the routers are provided and implementing wavelength conversion by providing a wavelength converter in at least some of the nodes, increases the wavelength reuse within the network and the flexibility of the network.
Full connectivity between all the nodes in the network may be realized by splitting the infrastructure of a two-dimensional optical network in two physical layers and then interconnecting the two layers through the routers making a three-dimensional network.
In accordance with an eighth aspect of the invention there is provided a method for interconnecting at least two optical networks, the optical networks being of the type defined above. The method includes the step of using a network layer comprising reconfigurable wavelength routing network elements in at least one of the nodes as a main network layer, and using the optical networks as sub-networks of the main network, the connections between the main network and the sub-networks being provided in at least one of the nodes of the main network. Preferably optical cross-connects (OXC) are used as the reconfigurable wavelength routing network elements in the nodes of the main network.
Recapitulating, the present invention is directed to a new method of routing wavelength bands. Conventional WDM systems, network elements and networks, are designed to handle/route individual wavelengths. Although many individual wavelength channels can be handled at a time in conventional networks, channels are still handled individually and are de-multiplexed and multiplexed and routed individually. Instead, the present invention proposes routing individual channels and/or wavelength bands that consist of more than one individual wavelength channel, by the components (network elements) and/or by the network. This routing can be accomplished for any type of optical network (any topology or architecture).
The present invention is further directed to a new network and a new method for routing signals within an optical network, wherein the network includes a mesh all-optical network using passive wavelength routing at the node. Passive components provide a mesh network of virtual (i.e. possible) direct paths for end-to-end connections between the nodes of a network. Depending on the available fibre infrastructure that connects these nodes, a mesh of virtual paths is created by:
a) allowing only a selection of possible xe2x80x9cflowsxe2x80x9d for all signals in geometrical terms (e.g. only transmissions towards S, E, S-followed-by-E are possible etc.), and
b) applying wavelength dependent rules for these signals as well as flow-direction dependent rules (i.e. the same wavelength may follow a different path if it enters a node from S than if it had entered the node from E).
Such a network may be considered as a passive medium with possible end-to-end paths where the intermediate nodes arexe2x80x94logicallyxe2x80x94invisible. Which paths are in use is decided by the management system.
A regular grid network is a simple example of a mesh network. Routers can be placed at locations that do not correspond to nodes in order to simplify the design in the general case. The present invention can be applied to any topology.
An optical network according to the present invention may include for example:
1. Fixed wavelength lasers and fixed receivers. This embodiment provides some flexibility by choosing different lasers at the transmitter side. In this embodiment, the routing function is performed electrically at the transmitter side.
2. Arrays of lasers and fixed receivers.
3. Arrays of lasers or fixed lasers and tunable receivers.
4. Tunable lasers or arrays of lasers and fixed receivers. This embodiment offers a lot of re-configurability, restoration, and allocation of bandwidth on demand.
5. Tunable lasers or arrays of lasers and tunable receivers. This embodiment offers maximum flexibility.
The xe2x80x9crulesxe2x80x9d mentioned above can be fixed. However, the network flexibility can be increased by making the rules xe2x80x9cre-configurablexe2x80x9d. This may be achieved by adding:
1. Switching elements at the node (that switch the direction of the signal).
2. Wavelength conversion at the node.
Advantageously, the network of the present invention can scale beyond the limits that are defined by the maximum number of wavelengths by means of a hierarchical architecture. A separate top network can serve to interconnect a number of sub-networks, which work based on the principle of the present invention. The top network can be switched or switch-less.
As mentioned above, the present inveniton includes passive wavelength routers. In one embodiment, a passive wavelength router has a plurality of input fibres and a plurality of output fibres. The passive router can be built using conventional passive optical components and routes passively groups of wavelengths to its different outputs based on a set of predetermined rules that depend on:
a) the entering port of the signal, and/or
b) the wavelength of the signal
In the passive router of the present invention, signals are not cross-connected i.e. signals from different inputs that are carried by the same optical wavelength (or frequency) are not exclusively directed to different outputs. In other words, signals from two (or more) different input ports that are carried by the same optical wavelength (or frequency) may be directed to the same output. Which of two or more such xe2x80x9ccollidingxe2x80x9d virtual paths is active, can be determined by a management system.
Two examples of the above router are later discussed for the case of a grid network. In one example the signals from some of the inputs are added and then routed on. In another example the input ports are treated individually and then several outputs are combined.
The drop function can take place individually at each input port. Alternatively, many input ports may be combined, and the drop function can be performed on the composite.
The add function can take place individually at each output, after routing has otherwise taken place. Alternatively, the add fucntion can take place before (at least part of) the routing has taken place for a combination of outputs.
As mentioned above, switching elements can be added to add some re-configurability to the functionality of the router. Similarly, wavelength conversion elements can be added to add some re-configurability to the functionality of the router. In one embodiment, the router routes bands of several consecutive wavelength channels.
Packet switching or cell-routing can be realised over the network of the present invention. IP, ATM, etc, can be carried directly over WDM using the network of the present invention. The optical network as stated above and the method for implementing an all-optical network as stated above may be used for providing a national backbone network, a Regional, a Metropolitan or an Access Network.